Molybdenum compounds of high purity are required for various applications such as in catalysts, etc.
Heretofore, there have been a number of processes for upgrading relatively impure molybdenum compounds as molybdenum oxide.
U.S. Pat. No. 3,957,946 discloses a process for purifying moybdenum which involves subjecting impure concentrates of molybdenum oxide to oxidative roasting, followed by ammoniacal leaching of the molybdic oxide to produce ammonium molybdate which is further purified by passage through a chelating cation exchange resin.
U.S. Pat. Nos. 3,848,049 and 3,829,550 disclose purification processes for molybdenum which involve leaching of molybdenum oxide with dilute nitric acid.
U.S. Pat. No. 3,694,147 discloses a process for purifying molybdenum oxide of impurities such as lead, copper, iron, zinc, and some alkaline earths by leaching with an aqueous solution of a hydrohalic acid and an ammonium salt of the halide. This patent does not teach removal of potassium. It is specifically stated in this patent that nitric acid-ammonium nitrate are unsatisfactory in the process of the invention.
U.S. Pat. No. 3,860,419 relates to a process for recovering molybdenum from molybdenum concentrates by digesting the concentrate in an aqueous nitric acid solution containing ammonium nitrate, separating the solid phase from the liquid phase, and treating the solid phase with ammonium hydroxide to produce ammonium molybdate. In column 3, lines 9 to 16, the digestion solution is disclosed as containing substantially 1.0 to 4.0 moles per liter of free nitric acid and 0.5 to 2.0 moles per liter of ammonium nitrate. The weight ratio of solution to concentrate is substantially 1 to 1 to 3 to 1. In column 3, lines 17 to 26, the preferred range of the solution is described as containing from about 2.0 to 2.3 moles per liter nitric acid and 1.5 to 2 moles per liter of ammonium nitrate. Weber teaches the use of nitric acid-ammonium nitrate for removal of "alkali metals". There is no mention in Weber of removal of potassium.
In the above processes, some impurities remain to contaminate the product.
Potassium is a particularly difficult impurity to remove. When U.S. Pat. No. 3,860,419 is followed for purifying molybdenum, potassium is not sufficiently reduced for use as high purity material.
Therefore, a process for purifying molybdenum in which the potassium level is further reduced would be highly desirable and an advancement in the art.